The effect of different weeding frequencies and plant density on growth and yield of sunflower (helianthus annuus)
The events that take place during the postnatal morphogenesis of the lung of the quokka wallaby, Setonix brachyurus, were investigated using the light microscope and both the scanning and transmission electron microscopes. The lung of the 3-day joey was at late canalicular stage and comprised large airways and tubules separated by thick mesenchymal interstitium. The tubules were lined by a low cuboidal epithelium but had few portions with true gas exchange barrier where capillaries came into close contact with a squamous type of epithelium. By the fifth day post partum, the lung entered the early saccular stage characterised by large air sacs, thinner septa, a better developed double capillary system and conversion of the cuboidal epithelium into a squamous one of type I cells interrupted by groups of cuboidal type II cells with lamellar bodies. Transitory respiratory bronchioles were recognisable towards the end of this stage. Formation of secondary septa was observable by day 15, and divided the saccules into several generations of smaller air spaces. There were alternating and concurrent periods of tissue proliferation and air space expansion, followed by septal thinning. Alveolization started from about 125 days postpartum when the first burst of small sized air spaces bounded by septa with a single capillary layer were encountered. By day 180 the process of alveolization was completed with only occasional septa showing a double capillary system and by day 210 postnatally, the lung resembled that of an adult. For the first time in a mammal, the canalicular stage was encountered postnatally during lung development.